A mathematical approach, based on the discriminant analysis of the fluctuations of the voltage signals, is proposed for the first time to identify the boundary between the particulate and the circulating three-phase fluidization regimes. In this new method, the particle size, liquid viscosity, gas flow rate, superficial liquid velocity, and solid circulating rate, which significantly affect on the characteristics of the voltage signals, are selected as the discriminant variables. The new approach proved to be a reliable method to determine the transition velocity between the particulate and the circulating fluidization regimes. The effects of particle size, liquid viscosity, and gas flow rate on the transition velocity are investigated as well. © 2004 Elsevier B.V. All rights reserved.